Nanogenerator takes charge from motion

Portland, Ore. -- The first nanoscale piezoelectric generator, which could one day use environmental motion to provide unlimited electricity for small devices, has been demonstrated by researchers in Georgia.

"Ours is the first work achieving nanoscale energy conversion," said Zhong Lin Wang, professor at the School of Materials Science and Engineering at the Georgia Institute of Technology (Atlanta). "Our piezoelectric nanogenerators convert vibration energy, from acoustic or ultrasonic waves, or hydraulic energy, from body fluids or blood flow, into electric energy that can be used to power small devices without requiring a battery."

For the demonstration, Wang grew arrays of nanoscale zinc-oxide piezoelectric nanowires perpendicular to a sapphire substrate and coupled the material's piezoelectric and semiconducting properties. While Wang's group has not yet created a batteryless device, it did use an atomic-force microscope (AFM) to demonstrate how mechanical motion of the arrays of piezoelectric semiconductors could initiate a charge cycle for future batteryless devices.

"We have the first demonstration that shows how the coupling of piezoelectric and semiconducting properties can control a charge-and-discharge process for wireless devices like biomedical sensors, which could be implanted and self-powered without using batteries," said Wang.

While the demonstration shows that the piezoelectric nanogenerator remains years away from powering commercial devices, the talk focused on its great potential. Converting mechanical energy into electricity with nanoscale piezoelectric materials could provide the foundation for future wireless applications, the researchers suggested. Besides simplifying medical implants, piezoelectric nanogenerators could also power remote sensors or even recharge conventional batteries. For instance, the researchers envision soldiers in the field with nanogenerators built into their uniforms so that their normal body movements can automatically recharge their communication devices' batteries.

"Nanogenerators could be the foundation for in situ, real-time and implantable biosensing, biomedical monitoring and biodetection, and have great potential for defense and civil applications," said Wang. He added that nanogenerators "can also be applied to building wireless, self-powered sensors by harvesting energy from the environment."

Piezoelectric materials were discovered by Pierre and Jacques Curie, who first showed how to generate electricity from motion in 1880, creating the world's first piezoelectric sensor. The converse effect--transducing motion from applied electricity, as in flat piezoelectric speakers--was demonstrated by the brother inventors the following year. These piezoelectric demonstrations convinced France's military scientists to create the world's first ultrasonic submarine detector in 1917--the precursor to sonar.

In those days, piezoelectric materials had to be mined from quartz crystals, which are still used as frequency references and elastic sensors and in acoustic holography to detect microscopic flaws in structures. However, today's piezoelectric materials can be created in the laboratory, enabling a variety of synthesized compounds, from ceramic transducers to microphones to accelerometers to actuators to surface acoustic-wave filters. Synthesized piezoelectric materials are also offered as solid-state replacements for solenoids and can act as electrostatic "muscles" for microelectromechanical systems.

The material Wang favors is zinc oxide, which acts as a piezoelectric semiconductor because of asymmetries in its crystalline structure. The single-crystal zinc-oxide molecules always align their oxygen end against the substrate, with the zinc end always sticking up perpendicular to the surface. Since zinc is positively charged, while the oxygen ion is negatively charged, the result is an electrostatic polarization--a perpendicular dipole moment all along a piezoelectric surface.

The permanent polarization of piezoelectric materials enables their unique ability to generate electricity when deformed--as when used as a sensor--as well as to be physically deformed when driven by external electricity, as in a piezoelectric speaker. Basically, bending the material concentrates a charge on one side and spreads out the opposite charge on the opposing surface, enabling the inherent ac operation of piezoelectric materials.

In 2003, Wang demonstrated that zinc-oxide piezoelectric materials could be fashioned into nanosprings. He fabricated them from high-purity zinc-oxide powder deposited using a solid-vapor evaporation process in vacuum at 1,350° Celsius, in the presence of argon gas. Wang's nanosprings measured just 10 to 60 nanometers wide and 5 to 20 nm thick, with their coils measuring 500 to 800 nm in diameter and up to several hundreds of microns in length. He demonstrated that functionalizing them to sense particular substances could enable them to be used in medical diagnostics.

Wang used the same zinc-oxide material for his current research and fashioned it into arrays of nanowires grown perpendicular to a sapphire substrate. The nanowires were 200 to 500 nm high and 20 to 40 nm wide. By seeding the sapphire substrate with gold nanoparticles, which served as catalysts, the nanowires were grown in a array with a 100-nm pitch. Whenever the nanowires are deflected, they concentrate charge on the side toward the bend, and when released, they create a current as they wave back and forth.

Wang confirmed their electrical-generating capabilities with an atomic-force microscope, which he pushed against the nanowires to deform them. When the moving tip slipped off the end of the deformed nanowire, it vibrated back and forth, generating a current.

The nanowires proved durable enough to be bent as much as 50° without breaking. Wang estimates that about 30 percent of the input mechanical energy could be converted into electrical energy.

Wang also holds faculty positions at Peking University and the National Center for Nanoscience and Technology of China. He performed the work at the Georgia Institute of Technology with doctoral candidate Jinhui Song.

Wang's next step will be to create a real power-generating material that can scale up to tackle bigger power capacities. Small devices requiring only a few microamps could be powered by nanogenerator arrays as small as 10 microns square, Wang said.

Currently, however, he wants to show that larger arrays of nanogenerators can be ganged together to tackle bigger power requirements.